Methacrylic resins have excellent transparency, surface hardness, and so forth, and only display a small degree of the optical property of birefringence. For these reasons, methacrylic resin has been receiving attention as optical resin for optical material in various optical products such as flat panel displays, e.g. liquid crystal displays, plasma displays, and organic EL displays, small infrared sensors, fine optical waveguides, micro lenses, and DVD/Blu-ray Disc pickup lenses handling short-wavelength light, optical discs, optical films, and plastic substrates. The market for methacrylic resin is therefore expanding.
Particularly, methacrylic resin having a cyclic structure-containing main chain and compositions containing such methacrylic resin are known to have excellent performance in both heat resistance and optical property (e.g. see WO 2011/149088 A1 (PTL 1)), and the demand for them has been rapidly growing year by year. However, methacrylic resin having a cyclic structure-containing main chain and exhibiting improved heat resistance and optical property as mentioned above sometimes encounters problems such as coloring and decreased transmittance due to absorption of light in a visible light range caused by the ring structure or the like. In view of this, methods of reducing residual unreacted cyclic monomers in methacrylic resin have been disclosed in order to obtain methacrylic resin having a cyclic structure-containing main chain and exhibiting high transparency with little coloring.
For example, JP H9-324016 A (PTL 2) proposes a method of obtaining heat-resistant methacrylic resin having excellent transparency with little coloring by, in a manufacturing method of feeding a part of a monomer component including N-substituted maleimide (a) and methacrylic acid ester (b) to start polymerization and then feeding the remaining part of the monomer component during the polymerization, performing such control that causes the ratio of N-substituted maleimide (a) in the unreacted monomer component existing in the reaction system at the end of the feeding of the monomer component to be lower than the ratio of N-substituted maleimide (a) in the total feed of the monomer component, thus reducing residual N-substituted maleimide monomer content.
JP 2001-233919 A (PTL 3) proposes a method of suppressing coloring by, in a methacrylic acid ester-based monomer/maleimide monomer polymerization system using a sulfur-based chain transfer agent such as mercaptan, causing an acidic substance to exist in the reaction system, thus reducing a residual maleimide monomer and a maleimide monomer generated due to heating during molding processing or the like.